This is a list of this week's papers on quantum foundations published in the various journals or uploaded to the preprint servers such as arxiv.org and PhilSci Archive.

Testing foundations of quantum mechanics with photons. (arXiv:1501.03713v1 [quant-ph])

on 2015-1-16 9:08am GMT

Authors: Peter Shadbolt, Jonathan C. F. Matthews, Anthony Laing, Jeremy L. O'Brien

The foundational ideas of quantum mechanics continue to give rise to counterintuitive theories and physical effects that are in conflict with a classical description of Nature. Experiments with light at the single photon level have historically been at the forefront of tests of fundamental quantum theory and new developments in photonics engineering continue to enable new experiments. Here we review recent photonic experiments to test two foundational themes in quantum mechanics: wave-particle duality, central to recent complementarity and delayed-choice experiments; and Bell nonlocality where recent theoretical and technological advances have allowed all controversial loopholes to be separately addressed in different photonics experiments.

Bell on Bell's theorem: The changing face of nonlocality. (arXiv:1501.03521v1 [quant-ph])

on 2015-1-16 9:08am GMT

Authors: Harvey R. Brown, Christopher G. Timpson

Between 1964 and 1990, the notion of nonlocality in Bell's papers underwent a profound change as his nonlocality theorem gradually became detached from quantum mechanics, and referred to wider probabilistic theories involving correlations between separated beables. The proposition that standard quantum mechanics is itself nonlocal (more precisely, that it violates `local causality') became divorced from the Bell theorem per se from 1976 on, although this important point is widely overlooked in the literature. In 1990, the year of his death, Bell would express serious misgivings about the mathematical form of the local causality condition, and leave ill-defined the issue of the consistency between special relativity and violation of the Bell-type inequality. In our view, the significance of the Bell theorem, both in its deterministic and stochastic forms, can only be fully understood by taking into account the fact that a fully Lorentz-covariant version of quantum theory, free of action-at-a-distance, can be articulated in the Everett interpretation.

Space-time mechanics: Quantum causal structure and expansive force. (arXiv:1501.03644v1 [hep-th])

on 2015-1-16 9:07am GMT

Authors: Mauricio Valenzuela

Combining twistor space and phase space formulation of quantum mechanics we propose a new framework of quantization of geometries which incorporates Wigner functions for geometrical observables. Quantizing the light-cone in 2+1D and 3+1D results in one-sheet "quantum hyperboloids". We propose that the latter rule the causal structure of the space-time, yielding uncertainty of positions and space-time curvature. The quantum hyperboloid predicts accelerated propagation of signals and effective space expansion. These effects are noticeable at scales of the quantization parameter in twistor space and negligible at much larger scales since the hyperboloid is asymptotic to the light-cone. Due to space-time non-commutativity it is necessary to introduce notions of observers which are able to determine distances in specific directions. Thus, in the perspective of a time-observer, time and radius of spatial sections of the quantum hyperboloid become discrete and bounded from below. Hence the time is quantized and punctual events are replaced by quantum events, which at determined time occur in a finite region of space bounded by quantum circles or quantum spheres respectively in 2+1D and 3+1D dimensions. Inside the latter bounds there is no notion of time and causality. We claim that quantum spheres may prevent a singular origin of the universe and complete evaporation of black holes. Using a twistor-space/phase-space correspondence allows us to compute the entropy of a finite volume in twistor-phase-space and observe that it is proportional to the area of a sphere in space-time. The quantum fluctuations of light-rays in 3+1 dimensions generate planes through the origin of the space-time, whose whole set is equivalent to the quantum version of the Grassmannian Gr(2,4). We show that these quantum algebraic variaties enjoy infinite symmetries known as higher spin symmetries.

A general perspective on time observables

on 2015-1-15 9:37am GMT

Publication date: August 2014

**Source:**Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, Volume 47

Author(s): Bryan W. Roberts

I propose a general geometric framework in which to discuss the existence of time observables. This framework allows one to describe a local sense in which time observables always exist, and a global sense in which they can sometimes exist subject to a restriction on the vector fields that they generate. Pauli׳s prohibition on quantum time observables is derived as a corollary to this result. I will then discuss how time observables can be regained in modest extensions of quantum theory beyond its standard formulation.

Can the ontological models framework accommodate Bohmian mechanics?

on 2015-1-15 9:37am GMT

Publication date: November 2014

**Source:**Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, Volume 48, Part A

Author(s): Benjamin Feintzeig

The ontological models framework has been proposed as a tool to prove general results about many competing interpretations of quantum mechanics at once. I argue that the ontological models framework is at best ambiguous, and at worst unable to accomplish its task of representing even the most well known interpretations of quantum mechanics. I show that when the framework is made mathematically precise, it cannot accommodate Bohmian mechanics, a well known interpretation of quantum mechanics in terms of hidden variables.

Causality and chance in relativistic quantum field theories

on 2015-1-15 9:37am GMT

Publication date: November 2014

**Source:**Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, Volume 48, Part B

Author(s): Richard A. Healey

Bell appealed to the theory of relativity in formulating his principle of local causality. But he maintained that quantum field theories do not conform to that principle, even when their field equations are relativistically covariant and their observable algebras satisfy a relativistically motivated microcausality condition. A pragmatist view of quantum theory and an interventionist approach to causation prompt the reevaluation of local causality and microcausality. Local causality cannot be understood as a reasonable requirement on relativistic quantum field theories: it is unmotivated even if applicable to them. But microcausality emerges as a sufficient condition for the consistent application of a relativistic quantum field theory.

Response to Bryan Roberts: A new perspective on T violation

on 2015-1-15 9:37am GMT

Publication date: Available online 25 July 2014

**Source:**Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics

Author(s): Abhay Ashtekar

It is surprising that the fundamental, microscopic laws of Nature are not invariant under time reversal. In his article, Three Merry Roads to T-Violation, Dr. Bryan Roberts provided a succinct summary of the theoretical frameworks normally used to interpret the results of the experiments that established this fact. They all rely on the detailed structure of quantum mechanics. In this ‘response’ to Dr. Robert׳s talk, I will show that these experiments can be interpreted using a much more general framework. Consequently, should quantum mechanics be eventually replaced by a new paradigm, e.g., because of quantum gravity, these experiments could still be used to argue that the microscopic laws violate T invariance.

Response to Dr. Pashby: Time operators and POVM observables in quantum mechanics

on 2015-1-15 9:37am GMT

Publication date: Available online 22 September 2014

**Source:**Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics

Author(s): Gordon N. Fleming

I argue against a general time observable in quantum mechanics except for quantum gravity theory. Then I argue in support of case specific arrival, dwell and relative time observables with a cautionary note concerning the broad approach to POVM observables because of the wild proliferation available.

on 2015-1-15 9:37am GMT

Publication date: November 2014

**Source:**Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, Volume 48, Part A

Author(s): Chris Heunen

Entanglement and disentanglement in relativistic quantum mechanics

on 2015-1-15 9:37am GMT

Publication date: November 2014

**Source:**Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, Volume 48, Part B

Author(s): Jeffrey A. Barrett

A satisfactory formulation of relativistic quantum mechanics requires that one be able to represent the entangled states of spacelike separated systems and describe how such states evolve. This paper presents two stories that one must be able to tell coherently in order to understand relativistic entangled systems. These stories help to illustrate why one׳s understanding of entanglement in relativistic quantum mechanics must ultimately depend on the details of one׳s strategy for addressing the quantum measurement problem.

The problem of confirmation in the Everett interpretation

on 2015-1-15 9:37am GMT

Publication date: August 2014

**Source:**Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, Volume 47

Author(s): Emily Adlam

I argue that the Oxford school Everett interpretation is internally incoherent, because we cannot claim that in an Everettian universe the kinds of reasoning we have used to arrive at our beliefs about quantum mechanics would lead us to form true beliefs. I show that in an Everettian context, the experimental evidence that we have available could not provide empirical confirmation for quantum mechanics, and moreover that we would not even be able to establish reference to the theoretical entities of quantum mechanics. I then consider a range of existing Everettian approaches to the probability problem and show that they do not succeed in overcoming this incoherence.

Maudlin׳s challenge refuted: A reply to Lewis

on 2015-1-15 9:37am GMT

Publication date: August 2014

**Source:**Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, Volume 47

Author(s): Ruth E. Kastner

Lewis has recently argued that Maudlin׳s contingent absorber experiment remains a significant problem for the Transactional Interpretation (TI). He argues that the only straightforward way to resolve the challenge is by describing the absorbers as offer waves, and asserts that this is a previously unnoticed aspect of the challenge for TI. This argument is refuted in two basic ways: (i) it is noted that the Maudlin experiment cannot be meaningfully recast with absorbers described by quantum states; instead the author replaces it with an ordinary which-way experiment; and (ii) the extant rebuttals to the Maudlin challenge in its original form are not in fact subject to the alleged flaws that Lewis ascribes to them. This paper further seeks to clarify the issues raised in Lewis’ presentation concerning the distinction between quantum systems and macroscopic objects in TI. It is noted that the latest, possibilist version of TI (PTI) has no ambiguity concerning macroscopic absorbers. In particular, macroscopic objects are not subject to indeterminate trajectories, since they are continually undergoing collapse. It is concluded that the Maudlin challenge poses no significant problem for the transactional interpretation.

Four tails problems for dynamical collapse theories

on 2015-1-15 9:37am GMT

Publication date: February 2015

**Source:**Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, Volume 49

Author(s): Kelvin J. McQueen

The primary quantum mechanical equation of motion entails that measurements typically do not have determinate outcomes, but result in superpositions of all possible outcomes. Dynamical collapse theories (e.g. GRW) supplement this equation with a stochastic Gaussian collapse function, intended to collapse the superposition of outcomes into one outcome. But the Gaussian collapses are imperfect in a way that leaves the superpositions intact. This is the tails problem. There are several ways of making this problem more precise. But many authors dismiss the problem without considering the more severe formulations. Here I distinguish four distinct tails problems. The first (bare tails problem) and second (structured tails problem) exist in the literature. I argue that while the first is a pseudo-problem, the second has not been adequately addressed. The third (multiverse tails problem) reformulates the second to account for recently discovered dynamical consequences of collapse. Finally the fourth (tails problem dilemma) shows that solving the third by replacing the Gaussian with a non-Gaussian collapse function introduces new conflict with relativity theory.

The primitive ontology of quantum physics: Guidelines for an assessment of the proposals

on 2015-1-15 9:37am GMT

Publication date: August 2014

**Source:**Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, Volume 47

Author(s): Michael Esfeld

The paper seeks to make progress from stating primitive ontology theories of quantum physics—notably Bohmian mechanics, the GRW matter density theory and the GRW flash theory—to assessing these theories. Four criteria are set out: (a) internal coherence; (b) empirical adequacy; (c) relationship to other theories; and (d) explanatory value. The paper argues that the stock objections against these theories do not withstand scrutiny. Its focus then is on their explanatory value: they pursue different strategies to ground the textbook formalism of quantum mechanics, and they develop different explanations of quantum non-locality. In conclusion, it is argued that Bohmian mechanics offers a better prospect for making quantum non-locality intelligible than the GRW matter density theory and the GRW flash theory.

Against the empirical viability of the Deutsch–Wallace–Everett approach to quantum mechanics

on 2015-1-15 9:37am GMT

Publication date: August 2014

**Source:**Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, Volume 47

Author(s): Richard Dawid , Karim P.Y. Thébault

The subjective Everettian approach to quantum mechanics presented by Deutsch and Wallace fails to constitute an empirically viable theory of quantum phenomena. The decision theoretic implementation of the Born rule realized in this approach provides no basis for rejecting Everettian quantum mechanics in the face of empirical data that contradicts the Born rule. The approach of Greaves and Myrvold, which provides a subjective implementation of the Born rule as well but derives it from empirical data rather than decision theoretic arguments, avoids the problem faced by Deutsch and Wallace and is empirically viable. However, there is good reason to cast doubts on its scientific value.

Berry phase and quantum structure

on 2015-1-15 9:37am GMT

Publication date: November 2014

**Source:**Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, Volume 48, Part A

Author(s): Holger Lyre

The paper aims to spell out the relevance of the Berry phase in view of the question what the minimal mathematical structure is that accounts for all observable quantum phenomena. The question is both of conceptual and of ontological interest. While common wisdom tells us that the quantum structure is represented by the structure of the projective Hilbert space, the appropriate structure rich enough to account for the Berry phase is the U(1) bundle over that projective space. The Berry phase is ultimately rooted in the curvature of this quantum bundle, it cannot be traced back to the Hamiltonian dynamics alone. This motivates the ontological claim in the final part of the paper that, if one strives for a realistic understanding of quantum theory including the Berry phase, one should adopt a form of ontic structural realism.

Waiting for the quantum bus: The flow of negative probability

on 2015-1-15 9:37am GMT

Publication date: November 2014

**Source:**Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, Volume 48, Part A

Author(s): A.J. Bracken , G.F. Melloy

It is 45 years since the discovery of the peculiar quantum effect known as ‘probability backflow’, and it is 20 years since the greatest possible size of the effect was characterized. Recently an experiment has been proposed to observe it directly, for the first time, by manipulating ultra-cold atoms. Here a non-technical description is given of the effect and its interpretation in terms of the flow of negative probability.

Measurements according to Consistent Histories

on 2015-1-15 9:37am GMT

Publication date: November 2014

**Source:**Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, Volume 48, Part A

Author(s): Elias Okon , Daniel Sudarsky

We critically evaluate the treatment of the notion of measurement in the Consistent Histories approach to quantum mechanics. We find such a treatment unsatisfactory because it relies, often implicitly, on elements external to those provided by the formalism. In particular, we note that, in order for the formalism to be informative when dealing with measurement scenarios, one needs to assume that the appropriate choice of framework is such that apparatuses are always in states of well defined pointer positions after measurements. The problem is that there is nothing in the formalism to justify this assumption. We conclude that the Consistent Histories approach, contrary to what is claimed by its proponents, fails to provide a truly satisfactory resolution for the measurement problem in quantum theory.

on 2015-1-13 8:48am GMT

Authors: Charles T. Sebens, Sean M. Carroll

A longstanding issue in attempts to understand the Everett (Many-Worlds) approach to quantum mechanics is the origin of the Born rule: why is the probability given by the square of the amplitude? Following Vaidman, we note that observers are in a position of self-locating uncertainty during the period between the branches of the wave function splitting via decoherence and the observer registering the outcome of the measurement. In this period it is tempting to regard each branch as equiprobable, but we argue that the temptation should be resisted. Applying lessons from this analysis, we demonstrate (using methods similar to those of Zurek's envariance-based derivation) that the Born rule is the uniquely rational way of apportioning credence in Everettian quantum mechanics. In doing so, we rely on a single key principle: changes purely to the environment do not affect the probabilities one ought to assign to measurement outcomes in a local subsystem. We arrive at a method for assigning probabilities in cases that involve both classical and quantum self-locating uncertainty. This method provides unique answers to quantum Sleeping Beauty problems, as well as a well-defined procedure for calculating probabilities in quantum cosmological multiverses with multiple similar observers.

Bell Inequality and Many-Worlds Interpretation. (arXiv:1501.02691v1 [quant-ph])

on 2015-1-13 8:48am GMT

Authors: Lev Vaidman

It is argued that the lesson we should learn from Bell's inequalities is not that quantum mechanics requires some kind of action at a distance, but that it leads us to believe in parallel worlds.

Holographic Inflation and the Low Entropy of the Early Universe. (arXiv:1501.02681v1 [hep-th])

on 2015-1-13 8:47am GMT

Authors: Tom Banks

This is a completely rewritten version of the talk I gave at the Philosophy of Cosmology conference in Tenerife, September 2014, which incorporates elements of my IFT Madrid Anthropics Conference talk. The original was too technical. The current version uses intuitive notions from black hole physics to explain the model of inflationary cosmology based on the Holographic Space Time formalism. The reason that the initial state of the universe had low entropy is that more generic states have no localized excitations, since in HST, localized excitations are defined by constraints on the fundamental variables. The only way to obtain a radiation dominated era, is for each time-like geodesic to see an almost uniform gas of small black holes as its horizon expands, such that the holes evaporate into radiation before they collide and coalesce. Comparing the time slicing that follows causal diamonds along a trajectory, with the global FRW slicing, one sees that systems outside the horizon had to undergo inflation, with a number of e-folds fixed by the present and inflationary cosmological constants, and the black hole number density on FRW slices just after inflation ends. These parameters also determine the size of scalar and tensor metric perturbations and the reheat temperature of the universe. I sketch a class of explicit finite quantum mechanical models of cosmology, which have these properties. Physicists interested in the details of those models should consult a recent paper\cite{holoinflation3}.

Is Spacetime Countable?. (arXiv:1501.02671v1 [gr-qc])

on 2015-1-13 8:47am GMT

Authors: Sean Gryb

Is there a number for every bit of spacetime, or is spacetime smooth like the real line? The ultimate fate of a quantum theory of gravity might depend on it. The troublesome infinities of quantum gravity can be cured by assuming that spacetime comes in countable, discrete pieces which one could simulate on a computer. But, perhaps there is another way? In this essay, we propose a picture where scale is meaningless so that there can be no minimum length and, hence, no fundamental discreteness. In this picture, Einstein's Special Relativity, suitably modified to accommodate an expanding Universe, can be reinterpreted as a theory where only the instantaneous shapes of configurations count.

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